Bipolar transport in organic field-effect transistors:: Organic semiconductor blends versus contact modification

The achievement of bipolar transport is an important feature of organic semiconductors, both for a fundamental understanding of transport properties and for applications such as complementary electronic devices. We have investigated two routes towards organic field-effect transistors exhibiting bipolar transport characteristics. As a first step, ambipolar field-effect transistors are realized by mixtures of p-conducting copper-phthalocyanine (CuPc) and n-conducting buckminsterfullerene (C-60). As a second step, bipolar transport in copper-phthalocyanine is achieved by a modification of the gate dielectric in combination with a controlled variation of the electrode materials used for carrier injection. The analysis involves the determination of charge-carrier mobilities and contact resistances by a single curve analysis and by the transfer length method. Comparison of both types of samples indicates that percolation is a crucial feature in mixtures of both materials to achieve ambipolar carrier flow, whereas in neat films of one single material suitable contact modification allows for bipolar charge-carrier transport. In the latter case, the obtained electron and hole mobilities differ by less than one order of magnitude.